In the specification of Australian patent No 591,802, which is essentially the same as WIPO Publication No WO 88/01258, it is pointed out that it is well known that refractory materials cannot be, simultaneously, mechanically strong, dense (which implies good corrosion resistance) and thermal shock resistant. For example, if samples of a dense alumina refractory are heated in a furnace to progressively higher temperatures and then quenched in water to room temperature, and the mechanical strength of the quenched samples is measured, it will be found that at a critical temperature (which varies according to the size of the sample and the conditions under which the experiment is conducted), there is a sudden and significant reduction in the strength retained by the sample after the quenching. In one reported instance, experiments with samples of a dense alumina showed that:
(a) after quenching the samples to room temperature (about 20.degree. C.) following their heating in a furnace to temperatures below 150.degree. C., the modulus of rupture of the quenched samples remained high (about 230 MPa in a test run conducted by the present inventor); thus no deterioration of the mechanical strength of the sample had resulted from the quenching from 150.degree. C. to ambient temperature; PA1 (b) when the samples were quenched to room temperature after heating to a furnace temperature of about 150.degree. C., the modulus of rupture of the quenched samples dropped dramatically (to about 60 MPa in the test run), showing that the thermal stresses generated during the quench were sufficient to activate pre-existing surface flaws (cracks) in the alumina refractory, and these flaws had propagated catastrophically through the material, causing the sudden loss of strength of the quenched sample; and PA1 (c) when the samples were quenched to room temperature from temperatures in excess of about 240.degree. C., the strength of the quenched samples fell approximately exponentially as the temperature drop of the thermal quench increased, the modulus of rupture of the samples reaching a value of about 19 MPa in the test run when the furnace temperature was 400.degree. C. PA1 (a) a matrix of a refractory material; and PA1 (b) particles of a ceramic material dispersed in the matrix material, the dispersed particles occupying from 1.0 percent by volume to 40 percent by volume of the composite refractory material, each dispersed particle comprising an agglomerate of microcrystals which (i) are strongly bonded together, (ii) exhibit a strong thermal expansion anisotropy, and (iii) have a size such that cracks do not form spontaneously within the agglomerates during cooling from a temperature of about 1600.degree. C. to room temperature; the ceramic material and the refractory material of the matrix being mutually chemically inert in the temperature range of the use of the composite refractory material; PA1 (c) the matrix material is a polycrystalline refractory ceramic material containing a fraction of from 5 percent to 90 percent by volume of grains having a diameter in the range of from 15 micrometers to about 80 micrometers. PA1 (a) mixing together a powder of the matrix material, a powder of the polycrystalline ceramic material to be dispersed within the matrix material, and a fugitive binder; PA1 (b) allowing the mixture to dry; PA1 (c) granulating the dried powder mixture; PA1 (d) preforming the granulated powder into at least one billet by die pressing; PA1 (e) isostatically pressing the die pressed billet or billets to form a green billet or green billets for sintering; PA1 (f) heating the green billet or billets at a predetermined rate until a sintering temperature in the range from 1200.degree. C. to 1800.degree. C. is reached; PA1 (g) holding the green billet or billets at the sintering temperature for a period in the range from 0 to 5 hours; and PA1 (h) cooling the sintered billet or billets to ambient temperature;
To reduce the dramatic change of mechanical strength due to unstable crack propagation in the quenching process, the traditional approach has been to introduce porosity into the refractory material. This reduces the low temperature strength of the refractory but the effect of quenching from higher temperatures is less dramatic. For example, samples of the same alumina material that has been described above which contain 5 percent porosity had an inherent low temperature strength of about half that of the dense material, and the low temperature modulus of rupture was approximately 103 MPa. When the samples were heated to the critical temperature of 150.degree. C., quenching to room temperature reduced the strength of the material with 5 percent porosity, but the modulus of rupture of the quenched material was about 87 MPa. Quenching samples of this material to room temperature from a temperature of 400.degree. C. produced samples having a modulus of rupture of about 70 MPa.
A refractory brick which is high in alumina content and has a porosity in the range from 15 percent to 25 percent completely solves the thermal shock problem. The modulus of rupture of quenched samples of such a material varies substantially linearly from about 19 MPa when the material is quenched from low temperatures to about 17 MPa when it is quenched from a temperature of about 400.degree. C. The small loss in mechanical strength when the material is quenched from higher temperatures is due to stable crack propagation. It is clear that with the increase in porosity, the mechanical strength at low temperatures has been sacrificed, being only about one tenth of the strength of a dense commercial alumina ceramic material. However, an even more serious sacrifice in performance of the brick has also occurred. With a porous alumina refractory brick, the rate of slag erosion (corrosion) of the alumina increases exponentially with the increase in porosity.
The aforementioned specification of Australian patent No 591,802 (WIPO Publication No WO 88/01258) discloses a range of refractory materials which are reasonably strong, are resistant to thermal shock, and are also corrosion resistant (that is, they are quite dense materials). Those materials are composite refractory materials which have a porosity which does not exceed 12 percent and which comprise a matrix of a refractory material with particles of a ceramic material dispersed in the matrix material. The dispersed particles occupy from 1.0 percent by volume to 40 percent by volume of the composite refractory material, and each dispersed particle comprises an agglomerate of microcrystals which (i) are strongly bonded together, (ii) exhibit a strong thermal expansion anisotropy, and (iii) have a size such that cracks do not form spontaneously within the agglomerates during cooling from a temperature of about 1600.degree. C. to room temperature. The ceramic material and the refractory material of the matrix are, of course, mutually chemically inert in the temperature range of the use of the composite refractory material.
Although such composite refractory materials perform well in their intended environments, it has been recognised that that it would be advantageous to produce refractory materials which have even better performance parameters than those of the materials described in the specification of Australian patent No 591,802. In particular, it is desirable to provide refractory materials possessing improved thermal shock resistance.
Hence it is an objective of the present invention to provide refractory materials which have better thermal shock resistance properties than the materials which constitute the invention of Australian patent No 591,802.